I’m not an expert on swordfighting, and I probably never will be, but yeah, parrying edge-on would be a quick way to dull that edge. I do have several edge-on-edge parries, but it’s not something I strive for, and when possible, Masahiro parries with the flat of his sword.

Lindybeige had a youtube video on that a while back. I thought he made a fairly good case that edge on edge was the usual way of doing things. Not that edge on flat never happened, or even was terribly unusual, but it wasn’t the way things were generally expected to go.

He gave a lot of reasons why edge on edge might be advantageous, but clincher as far as I was concerned was the historical evidence, chiefly hilt design. For instance, the quillons, intended primarily for hand protection, are always along the edges (when they’re there at all). That is, the edges of the swords are where they were most afraid of an opponents blade slicing down and chopping off your fingers.

True, edge on edge does mean more wear and tear on the blade, but so does shoving it through ribs and lopping off limbs and bashing skulls in and so on. Besides, if you’re too picky about things like that in the middle of a fight you’re liable to wind up dead. And as I’m sure Masahiro would agree, it’s easier to replace a sword than the hand wielding it.